Cause

The cause of this problem has been identified in Bug 8394351. It is caused by the adaptive fetch is not
getting kicked in for parallel group-by queries without aggregates.

Solution

This is fixed for RDBMS 11.2 release.
The Patch 8394351 is available at top of RDBMS 10g and 11.1 for many platforms.
If it needs a fix for others platforms,
please contact Oracle Support and request assistance for the Bug 8394351 problem.